When a large terrorist bomb is detonated near a building, flying glass from blown-out windows causes the most injuries. There is a clear need to harden windows in many government buildings to terrorist blast. The General Services Administration (GSA) plans to harden 800 existing government buildings over the next several years. Many new government buildings will also be glass hardened to blast. Depending on the perceived threat, GSA requirements are to protect about half the hardened windows against blasts of 4 psi and the other half against 10 psi.
Several window-hardening systems have been developed to eliminate or minimize the threat of injury from glass fragments produced by terrorist bombs. For new building construction, the typical approach is to use extra-thick glass (up to one inch) or to use strong ductile polymers such as Lexan. Retrofitting existing buildings with these approaches is often prohibitively expensive and, therefore, other approaches are used.
Current retrofit systems for hardening windows involve adding a film or curtain on the inside of the window to prevent the glass from flying into the room. The simplest retrofit system is to cover the inside of the window with polyester film over the portion of the glass inside the frame (so-called daylight film). Such films are already frequently used to limit ultraviolet light transmitted through the window. A blast wave impinging on glass covered with daylight film will still fracture the glass but, up to a limit, the film will adhere to the glass fragments. Furthermore, the film/glass structure raises the blast hardness compared to a bare window.
To further increase the blast hardness, the film may be anchored to the window using a variety of materials. Hardness can also be increased by a so-called horizontal catch bar mounted at the mid-height of the window. In this system, the film, with glass fragments still attached, is arrested by the catch bar as the film folds around it.
Current systems are insufficient for many applications because they concentrate the stress at the edge or at the middle of the film, thus limiting the blast pressure the film can withstand before it fails. Accordingly, there is a need in the art to develop a window-hardening method that relieves the stress on the film, has the flexibility to provide a range of hardness levels up to at least 10 psi, and is simple and inexpensive to install.